The present invention is directed to novel .beta.-lactams, a process for their preparation, and a process for the preparation of taxol involving the use of such .beta.-lactams.
The taxane family of terpenes, of which taxol is a member, has attracted considerable interest in both the biological and chemical arts. Taxol is a promising cancer chemotherapeutic agent with a broad spectrum of antileukemic and tumor-inhibiting activity, having the following structure: ##STR2## Because of this promising activity, taxol is currently undergoing clinical trials in both France and the United States.
The supply of taxol for these clinical trials is presently being provided by the bark from several species of yew. However, taxol is found only in minute quantities in the bark of these slow growing evergreens, causing considerable concern that the limited supply of taxol will not meet the demand. Consequently, chemists in recent years have expended their energies in trying to find a viable synthetic route for the preparation of taxols. So far, the results have not been entirely satisfactory.
One synthetic route that has been proposed is directed to the synthesis of the tetracyclic taxane nucleus from commodity chemicals. A synthesis of the taxol congener taxusin has been reported by Holton, et al. in JACS 110, 6558 (1988). Despite the progress made in this approach, the final total synthesis of taxol is, nevertheless, likely to be a multi-step, tedious, and costly process.
An alternate approach to the preparation of taxol has been described by Greene, et al. in JACS 110, 5917 (1988), and involves the use of a congener of taxol, 10-deacetyl baccatin III which has the structure shown below: ##STR3## 10-deacetyl baccatin III is more readily available than taxol since it can be obtained from the leaves of Taxus baccata. According to the method of Greene et al., 10-deacetyl baccatin III is converted to taxol by attachment of the C10 acetyl group and by attachment of the C13 .beta.-amido ester side chain through the esterification of the C-13 alcohol with a .beta.-amido carboxylic acid unit. Although this approach requires relatively few steps, the synthesis of the .beta.-amido carboxylic acid unit is a multi-step process which proceeds in low yield, and the coupling reaction is tedious and also proceeds in low yield. However, this coupling reaction is a key step which is required in every contemplated synthesis of taxol or biologically active derivative of taxol, since it has been shown by Wani, et al. in JACS 93, 2325 (1971) that the presence of the .beta.-amido ester side chain at C13 is required for anti-tumor activity.
A major difficulty remaining in the synthesis of taxol and other potential anti-tumor agents is the lack of a readily available unit which could be easily attached to the C13 oxygen to provide the .beta.-amido ester side chain. Development of such a unit and a process for its attachment in high yield would facilitate the synthesis of taxol as well as related anti-tumor agents having a modified set of nuclear substituents of a modified C13 side chain. This need has been fulfilled by the discovery of a new, readily available, side chain precursor chemical unit and an efficient process for its attachment at the C13 oxygen.